Hi, I'm Bryan Yeaton for The Weather Notebook. When your local meteorologist says that the dew points are pushing into the seventies, so it's going to be a sticky oneŠ does that make you scratch your head in wonder? Well, we're here to help, with a primer on dew point.
Since there is water in seemingly clear air, we can change that water from a gas to a visible liquid. How do we do that? By lowering its temperature. Every packet of air has a temperature, below which water vapor will condense to form visible droplets, which we call fog, or clouds. That temperature is the dew point. In order to have dew points in the 70's you have to have temperatures at least that warm, which also means the air can hold lots of water. That's why you feel so sticky.
Try this experiment on a day when no weather systems move through your area. Watch both the temperature and dew point over the course of a day. You'll find that during the cool of the morning, the humidity will be fairly high, and the dew point not too far from the actual temperature. But as the day warms and the air dries, the relative humidity will drop, but the dew point won't change very much. The holding capacity of the air has changed because it is warmer, but it will still condense at the same temperature. Cool the air down again at night, and you'll get dew, or some ground fog.
You can get hourly readings from your local radio to television station, The Weather Channel local forecasts, or from the National Weather Service. You can find your local office at www.nws.noaa.gov.
Our program is produced in New Hampshire by the Mount Washington Observatory, with help from the National Science Foundation.